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K-means Clustering with Dask: Image Filters for Cat Pictures


How to recreate an original cat image with least possible colors. An interesting use case of Unsupervised Machine Learning with K Means Clustering in Python.



By Luciano Strika, MercadoLibre
Picture of a kitten. It will be compressed using K means clustering.

Applying filters to images is not a new concept to anyone. We take a picture, make a few changes to it, and now it looks cooler. But where does Artificial Intelligence come in? Let’s try out a fun use case of Unsupervised Machine Learning with K Means Clustering in Python.

I’ve written before about K Means Clustering, so I will assume that you’re familiar with the algorithm. If you’re not, this is the in-depth introduction that I wrote.

Moreover, I also tried my hand at image compression (well, reconstruction) with autoencoders, to varying degrees of success.

However, this time, my goal is not to reconstruct the best possible image, but just to see the effects of recreating a picture with the least possible colors.

Instead of making the picture look as similar to the original as possible, I just want us to look at it and say “neat!”.

So how do we do this? I’m glad you asked.

How to do image filters with K-means Clustering

 
First of all, it’s always good to remember an image is just a vector of pixels. Each pixel is a tuple of three integer values between 0 and 255 (an unsigned byte), which represent that pixel’s color’s RGB values.

We want to use K-means clustering to find the k colors that best characterize an image. That just means we could treat each pixel as a single data point (in 3-dimensional space), and cluster them.

So first, we’ll want to turn an image into a vector of pixels in Python. Here’s how we do it.

As an aside, I don’t think the vector_of_pixels function needs to use a Python list. I’m sure there has to be some way to flatten a numpy array, I just couldn’t find any (at least not one that did it in the order I wanted).

If you can think of any way, let me know in the comments!

The next step is fitting the model to the image, so that it clusters the pixels into colors. Then, it’s just a matter of assigning the corresponding cluster color to each position in the image.

For instance, maybe our pic has only three colors: two reddish ones and a greenish one. If we fit that to 2 clusters, all the reddish pixels would turn some different shade of red (getting clustered together), and the other ones would turn into some greenish one.

But enough with the explanations, let’s see the program in action!
 
As usual, you are free to run it yourself with any pic you want, here’s the GitHub repository with the code.

The results

 
We will apply the filter to pictures of kittens, taken from the awesome “Cats vs Dogs” kaggle dataset.

We’ll start with a picture of a cat, and apply the filter with different values for k. Here’s the original picture:

Picture of a kitten. It will be compressed using K means clustering.

First, let’s check how many colors this picture originally had.

With just one line of Numpy, we count the unique values a pixel takes on this picture. This image in particular has 243 different colors, even though it has a total of 166167 pixels.

Now, let’s see the result of clustering it to 2, 5 and 10 different colors only.

cat image

With only two colors, all it’s doing is marking the darkest and lightest areas. This could however be useful if you’re an artist, drawing something in black and white (with ink, for instance) and would like to see what your reference’s outlines look like.
 

Figure

With only 5 different colors, the cat is already recognizable!

 

This kitten picture was compressed using K means Clustering.

The 10 colors picture may look a bit trippy, but it represents the original’s content pretty clearly.

Did you notice a trend? Each color we add has diminishing returns. The difference between having 2 colors and having 5, is a lot more than the difference between 5 and 10. However with 10 colors, the flat areas are smaller, and we have more granularity. Moving on to 15 and 24 colors!

Compressed picture of a kitten.

Compressed picture of a kitten.

Though it’s clear that the above image is filtered with 24 colors (10% of the original amount), we are representing the cat well enough and to a certain level of detail.

Moving on to a different picture: Here’s the original (256 different colors) and here’s a compressed one (24 colors again).

wo pictures of the same white kitten side by side. One was compressed using K means Clustering.

256 colors vs 24 colors. Notice any difference?

As an interesting note, the “compressed” image weighs 18KB and the uncompressed one 16KB. I don’t really know why this is, since compressors are pretty complicated beasts, but would love to read your theories in the comments.

Conclusions

 
We were able to make new images with only 10% of the original’s colors, which looked very similar to them. We also got some cool looking filters thanks to K means clustering. Can you think of any other fun application for clustering? Do you think other clustering techniques could have yielded more interesting results?

If you want to answer any of these questions, feel free to contact me on TwitterMedium or Dev.to.

Bio: Luciano Strika is a computer science student at Buenos Aires University, and a data scientist at MercadoLibre. He also writes about machine learning and data on www.datastuff.tech.

Original. Reposted with permission.

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